Liquid drop ejectors are used in a variety of industries to precisely and controllably dispense droplets of liquid. Drop ejectors can be used, for example, in the medical, chemical and printing industries.
Inkjet printing systems include one or more printheads that have arrays of drop ejectors that are controlled to make marks of particular sizes, colors, or densities in particular locations on the recording medium in order to print the desired image. In some types of inkjet printing systems the array(s) of dot forming elements extends across the width of the page, and the image can be printed one line at a time, as the recording medium is moved relative to the printhead. Alternatively, in a carriage printing system (whether for desktop printers, large area plotters, etc.) the printhead or printheads are mounted on a carriage that is moved past the recording medium in a carriage scan direction as the dot forming elements are actuated to make a swath of dots. At the end of the swath, the carriage is stopped, printing is temporarily halted and the recording medium is advanced. Then another swath is printed, so that the image is formed swath by swath.
An inkjet drop ejector includes a nozzle and a drop forming mechanism (such as a resistive heater for thermal inkjet, or a piezoelectric device for piezoelectric inkjet) in order to generate pressure within an ink-filled chamber and eject ink from the nozzle. In page-width inkjet printers as well as in carriage inkjet printers, the printhead and the recording medium are moved relative to one another as drops are ejected in order to form the image.
A limitation to how quickly an image can be printed is the refill time of the drop ejector. The refill time is the time required for the chamber to refill with ink so that a subsequent drop can be ejected after ejecting a previous drop. When a drop is ejected, a portion of the ink in the chamber exits the nozzle, and another portion of ink is pushed back toward the ink source. Capillary forces cause additional ink to refill the chamber from the ink source. Refill time depends on ink properties, such as surface tension and viscosity, as well as drop ejector geometries and surface properties, and operating conditions such as temperature. During refill, the ink meniscus approaches the nozzle opening, and can temporarily extend past the nozzle opening if the refill is underdamped. The refill time is the time such that the ink volume in the chamber is sufficiently replenished and the ink meniscus is sufficiently stabilized so that the drop volume and velocity of a subsequent drop of ink is similar to that of the previous drop of ink.
Refill time can be shortened by decreasing the volume of the drop of ink. However, in order to make sufficiently large spots of ink on the print medium to provide proper image quality, the drop volume is generally set to a particular drop volume or range of drop volumes. Refill time can also be shortened by increasing the surface tension and/or decreasing the viscosity of the ink. However, surface tension cannot be increased too much or the ink drops will not wick into the print medium sufficiently fast to achieve required dry times. In addition, viscosity cannot be decreased too much or the ink drops will not remain sufficiently localized in the location where they hit the print medium.
Consequently, a need exists for a method of operating a drop ejector that enables a short refill time to allow high frequency ejection of inks or other liquids having sufficiently large drop volume, sufficiently small surface tension, and sufficiently high viscosity.